It is important to detect a fuel injection condition, such as a fuel-injection-start timing, a fuel injection quantity and the like in order to accurately control an output torque and emission of an internal combustion engine. JP-2010-3004A (US-2009/0319157A1) and JP-2009-57924A (US-2009/0063013A1) describe that a fuel pressure sensor detects a variation in fuel pressure, which is caused in a fuel supply passage due to a fuel injection, whereby an actual fuel injection condition is detected.
For example, an actual fuel-injection-start timing is detected by detecting a timing at which the fuel pressure in the fuel injection system starts to be decreased due to the fuel injection. An actual fuel-injection-quantity is detected by detecting a decrease in fuel pressure due to the fuel injection. As above, if the actual fuel injection condition is detected, the fuel injection condition can be accurately controlled based on the detected fuel injection condition.
In a case that a multi-stage injection is performed during one combustion cycle, following matters should be noted. FIG. 5B shows a waveform (multi-stage injection waveform) “W” detected by a fuel pressure sensor while the multi-stage injection is performed. In this waveform “W”, a part of the waveform corresponding to the n-th fuel injection (refer to a portion enclosed by a dashed line in FIG. 5B) is overlapped with an aftereffect of the waveform corresponding to the m-th (m=n−1) fuel injection (refer to a portion enclosed by a dashed line in FIG. 5D).
In JP-2010-3004A, a model waveform “CALn−1” corresponding to only the m-th fuel injection is previously computed and stored as shown in FIG. 5D. Then, as shown in FIG. 5E, the model waveform “CALn−1” is subtracted from the detected waveform “W” to obtain a waveform “Wn” corresponding to only the n-th fuel injection. FIG. 5F shows this waveform “Wn”.
However, according to the present inventors' experiments, even if the model waveform “CALn−1” is simply subtracted from the detected waveform “W”, the waveform “Wn” is not obtained with high accuracy.